1. Field of the Invention
The invention is related to the field of communication networks and, in particular, to providing for improved charging for roaming users in IMS networks.
2. Statement of the Problem
Mobile communications allows for the situation where a mobile user is roaming. Roaming is a general term in mobile communications that refers to the extending of service in a location that is different from the home location where the service was registered. The term “roaming” originates from the GSM world where roaming is defined as the ability for a cellular customer to automatically make and receive voice calls, send and receive data, or access other services when traveling outside the geographical coverage area of the home network by means of using a visited network.
If a mobile user roams into a visited cellular network, such as a GSM network or a CDMA network, the Mobile Switching Center (MSC) in the visited cellular network serves calls for the roaming user. If a call is placed by the roaming user, the serving MSC collects charging information for the call, and generates a Charging Detail Record (CDR). The serving MSC then transmits the CDR to a billing system in the visited cellular network. The billing system then resolves the proper billing for the call. The billing system in the visited cellular network will also settle the billing with a billing system in the home cellular network based on an agreement between the two service providers. In a CDMA network, the billing systems use Cellular Intercarrier Billing Exchange Roamer (CIBER) to exchange charging information. In a GSM network, the billing systems use Transferred Account Procedure (TAP) to exchange charging information. When the billing is settled, the visited cellular network and the home cellular network will typically share revenues from the call.
One type of communication network gaining popularity is an IP Multimedia Subsystem (IMS) network. As set forth in the 3rd Generation Partnership Project (3GPP), IMS provides a common core network having a network architecture that allows for various types of access networks. The access network between a communication device and the IMS network may be a cellular network (e.g., CDMA or GSM), a WLAN (e.g., WiFi or WiMAX), an Ethernet network, or another type of wireless or wireline access network. The IMS architecture is initially defined by the 3GPP to provide multimedia services to communication devices over an Internet Protocol (IP) network, as IP networks have become the most cost savings bearer network to transmit video, voice, and data. Service providers are accepting this architecture in next generation network evolution.
Before a communication device receives service from an IMS network, the communication device attempts to register with the IMS network. To register according to 3GPP standards, the communication device transmits a register request message, such as a SIP REGISTER message, to a Proxy-Call Session Control Function (P-CSCF) through the appropriate access network. The P-CSCF identifies the home IMS network for the communication device, and then transmits another register request message to a Serving-Call Session Control Function (S-CSCF) in the home IMS network of the user. Responsive to the register request message, the S-CSCF generates an authentication request message, such as a Diameter Multimedia Authentication Request (MAR) message to retrieve the communication device's authentication data. The S-CSCF then authenticates the communication device using the appropriate authentication method. If the communication device is authenticated, then the S-CSCF generates a subscriber profile request message, such as a Diameter Server Assignment Request (SAR) message, and transmits the subscriber profile request message to the HSS. Responsive to the subscriber profile request message, the HSS identifies the subscriber profile for the user of the communication device. The HSS then transmits a response message, such as a Diameter Server Assignment Answer (SAA) message, to the S-CSCF that includes the subscriber profile for the user of the communication device. The S-CSCF may then process the subscriber profile to provide services for the communication device.
When a mobile user is roaming in the service area of a visited IMS network, the visited IMS network does not provide call control for sessions. If a roaming session is initiated, then the P-CSCF in the visited IMS network receives a session initiation message for the session (e.g., SIP INVITE message). The P-CSCF proxies the session control for the session to the S-CSCF in the home IMS network of the mobile user by forwarding the session initiation message to the S-CSCF. The S-CSCF in the home IMS network then provides session control for the session.
While providing session control, the S-CSCF in the home IMS network also provides charging control. At session initiation, the S-CSCF generates a home IMS Charging Identifier (ICID) for the session. The S-CSCF also transmits a start charging message, such as a Diameter Accounting Request (ACR[start]) message, to a Charging Data Function (CDF) in the home IMS network. Periodically during the session, the S-CSCF transmits interim charging messages, such as a Diameter ACR[interim] messages, to the CDF in the home IMS network. At the end of the session, the S-CSCF transmits a stop charging message, such as a Diameter ACR[stop] message, to the CDF in the home IMS network. The ACR messages all include the home ICID for the session so that the ACR messages may be correlated.
Based on the ACR[start,interim,stop] messages received from the S-CSCF, the CDF generates a Charging Data Record (CDR) for the session based on charging information included in the ACR messages. The charging information may include a service delivery start timestamp for the session, a service delivery stop timestamp for the session, a destination for the session, etc. The CDF then transmits the CDR to a billing system in the home IMS network. The billing system may then resolve any charging for the session based on the CDRs.
The P-CSCF in the visited IMS network may also provide some charging control for the session. The P-CSCF generates a visited IMS Charging Identifier (ICID) for the session. If the P-CSCF generates charging messages for the session, such as Diameter ACR[start,interim,stop] messages, then the P-CSCF transmits the ACR messages to a CDF in the visited IMS network. The ACR messages all include the visited ICID for the session so that the ACR messages may be correlated.
Based on the ACR[start,interim,stop] messages received from the P-CSCF, the CDF in the visited IMS network generates a CDR for the session based on charging information included in the ACR messages. The CDF then transmits the CDR to a billing system in the visited IMS network. The billing system may then resolve any charging for the session based on the CDRs.
One problem in present IMS networks is that there is no effective way to correlate charging information from a home IMS network and a visited IMS network. The S-CSCF in the home IMS network generates its own unique home ICID while the P-CSCF in the visited IMS network generates another unique visited ICID for the same session. Thus, the billing system in the home IMS network identifies the session based on a home ICID while the billing system in the visited IMS network identifies the same session based a visited ICID. Because of the different ICIDs for the same session, it is difficult for the two billing systems to correlate charging information generated in each IMS network and to settle a bill on the session.
Another problem with present IMS networks is that the P-CSCF in the visited IMS network may not be in the session at all, or may not have the appropriate or necessary charging information for the session. Because session control is being provided in the home IMS network, the home IMS network has the necessary charging information for the session. The P-CSCF may have some charging information, but may not have the necessary information to provide to the CDF to allow the CDF to generate a full CDR. As a result, the billing system in the visited IMS network may not be able to calculate a bill for the session. The service provider operating the visited IMS network has to rely on the billing system of the home IMS network to share the proper revenues for the session according to agreements in place. The service provider operating the visited IMS network unfortunately may not be able to verify the billing of the home IMS network or determine whether the home IMS network is sharing the appropriate revenues.